A series of ternary Eu 3+ complexes are presented consisting of a polydentate m-terphenyl-based Eu 3+ complex (Eu)1 and different antenna chromophores possessing lanthanide(III) ion coordinating properties. The series of investigated antenna chromophores consist of 1,10-phenanthroline, tetraazatriphenylene, and three β-diketonates, namely dibenzoylmethane, benzoyltrifluoroacetylacetonate, and hexafluoroacetylacetonate. As a result of the synergistic complexation of Eu 3+ by the polydentate ligand and the bidentate antenna, the distance between the antenna and lanthanide ion has been minimized and the Eu 3+ ion has been shielded completely from the solvent. These are two important requirements to obtain efficiently emitting lanthanide-(III) complexes. The formation of the ternary complexes and their photophysical properties, in particular the population of the Eu 3+ excited states and the efficiency of the sensitization process, have been studied in detail. Based on these measurements, it can be concluded that the aforementioned strategy of synergistic complexation has indeed led to the construction of efficiently emitting Eu 3+ complexes. The β-diketonate ternary Eu 3+ complexes combine a high stability (K ) 3.8 ( 0.2 × 10 7 M -1 ) with high overall luminescence quantum yields of up to 0.29. The energy transfer from the sensitizer to the Eu 3+ is exclusively to the 5 D 1 level, from which the 5 D 0 level is populated.

In general, sensitization of lanthanide(III) ions by organic sensitizers is regarded to take place via the triplet state of the sensitizers. Herein, we show that in dansyl- and lissamine-functionalized Nd3+ complexes energy transfer occurs from the singlet state of the sensitizers to the Nd3+ center. No sensitized emission was observed in the corresponding complexes with Er3+, Yb3+, and Gd3+ ions. Furthermore, the fluorescence of the sensitizers was quenched only in the Nd3+ complex and not in the complexes with the other ions. Only Nd3+ centers can accept energy from the singlet state of the dyes, because the excited states of Nd3+ have a high spectral overlap with the fluorescence of the dansyl and lissamine sensitizers, and because the selection rules allow a fast energy transfer, which apparently is competitive with the fluorescence.

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